The influence of methyl pentanoate (MP) addition to n-heptane on the species pool in a nonpremixed counterflow flame fueled with n-heptane at atmospheric pressure has been investigated experimentally and numerically. Two non-premixed flames in counterflow configuration have been examined: (1) n-heptane/Ar (5.3%/94.7%) vs O2/Ar (24.1%/75.9%) and (2) n-heptane/MP/Ar (2.5%/2.5%/95%) vs O2/Ar (19.6%/80.4%). Both flames had similar strain rates and stoichiometric mixture fractions to allow an adequate comparison of their structures. The mole fraction profiles of the reactants, major products, and intermediates in both flames were measured using flame sampling molecular beam mass spectrometry. These experimental data were used for validation of a detailed chemical kinetic mechanism, which was proposed earlier for prediction of combustion characteristics of n-heptane/iso-octane/toluene/MP mixtures. The addition of MP to n-heptane reduced the flame temperature and the peak mole fractions of many flame intermediates, responsible for the formation of polycyclic aromatic hydrocarbons, specifically, of benzene, cyclopentadienyl, acetylene, propargyl, and vinylacetylene. Significant discrepancies between the calculated and measured mole fractions of cyclopentadienyl and benzene were found. A kinetic analysis of the reaction pathways resulting in formation of these intermediates in both flames and a sensitivity analysis of cyclopentadienyl and benzene were carried out to understand the origins of the observed discrepancies. The peak mole fractions of the major flame radicals (H, O, OH, CH3) were found to decrease with MP addition. The influence of MP addition on the relative contributions of the primary stages of n-heptane consumption is discussed.